1. Field of the Invention
This invention relates to coating compositions which are cured in a two-stage curing process. The first cure is a photopolymerization UV cure, and the second cure is a heat or air drying cure, the second cure preferably being catalyzed by an acid catalyst generated in the initial cure step.
2. Description of the Prior Art
The combined impact of high energy costs and more stringent control regulations restricting emissions of volatile solvents into the atmosphere has created a need in the coatings industry for high solids or solventless systems which do not require a large amount of energy for conversion of the system into a high performance coating.
Coatings are known which offer 100% solids, rapid cure and low conversion energy demand. Examples are the UV or electron beam cured acrylated coatings. These materials, however, are considerably more expensive than solvent or water based coatings and additionally suffer from poor adhesion to many different types of substrates.
The rapid cure from such UV coatings is obtained by using a high concentration of reactive acrylic groups. The exchange of a double bond for two single bonds for every monomer polymerized can result in as much as a 20-30% reduction in volume for a complete polymerization reaction. In addition rapid cure in the absence of high heat, such as occurs in typical UV systems, results in little molecular stress relaxation and consequently poor adhesion to nonporous substrates. The high resin cost stems primarily from the required high concentration of acrylic monomer and the isocyanate or epoxide backbone required for reaction with the acrylic monomer.
UV cured coatings with backbones of the polyester or alkyd types might be employed to reduce shrinkage and reduce cost, but this also results in a decrease in cure response and degradation of cured film properties due to lower cross link densities.